1. Technical Field
This disclosure relates to a conductive member, a process cartridge using the conductive member, and an image forming device using the process cartridge.
2. Description of the Related Art
In an electrophotographic process such as that carried out in a copier, a laser printer, or a facsimile, a conductive member is conventionally used as a charging member, which performs a charging process on an image carrier (photoconductor), and a transfer member, which conducts a transfer process on toners on the photoconductor.
FIG. 1 is a schematic view illustrating an image forming device.
Referring to FIG. 1, the image forming device includes an image carrier 11 (photoconductor) onto which an electrostatic latent image is formed, a charging member 12 (charging roller: conductive member), which conducts a charging process in a contact state or a close state, a laser light 13 or an exposure light such as a reflection light of an original, a toner carrier 14 (development roller), which adheres toners 15 onto the electrostatic latent image of the image carrier, a transfer member 16 (transfer roller), which transfers the toner image on the image carrier to a recording medium 17, and a cleaning member 18 (blade), which cleans the image carrier after the transfer process. In addition, reference number 19 denotes toners removed from the image carrier by the cleaning member, and reference number 20 denotes a developing unit and reference number 21 denotes a cleaning unit.
In FIG. 1, functional units generally required for another electrophotographic process are not required for the present invention; thus, those are omitted.
The image forming device forms an image in the following order.
1. The charging roller 12 charges a surface of the photoconductor 11 at a predetermined potential.
2. An exposure unit (not shown) irradiates an image light to the photoconductor 11, so as to form an electrostatic latent image corresponding to a predetermined image on the photoconductor.
3. The development roller 14 develops the electrostatic latent image by the toners 15, so as to form a toner image (visualize a toner image) on the photoconductor 11.
4. The transfer roller 16 transfers the toner image on the photoconductor 11 onto a recording medium 17.
5. The cleaning unit 21 cleans the toners remaining on the photoconductor 11 without being transferred.
6. The recording paper 17 to which the toner image is transferred by the transfer roller 16 is fed to a fixing unit (not shown) in the arrow B direction. The fixing unit heats and presses the toners, so as to fix the toners onto the recording paper 17.
By repeating the above processes from 1 to 6, a predetermined image is formed on the recording paper 17.
As a charging method using the charging roller 12, a contact charging method, which brings the charging roller 12 into contact with the photoconductor 11, is known (for example, refer to JP S63-149668A, JP H01-211779A, and JP H01-267667A). However, the contact charging method has the following problems.
1. Charging roller track: The component of the charging roller exudes from the charging roller, and then adheres onto the surface of the photoconductor. If this adhesion is developed, the track of the charging roller remains on the surface of the photoconductor.
2. Charging noise: When applying an alternating voltage to the charging roller, the charging roller which has contact with the photoconductor vibrates, causing charging noise.
3. The decrease in the charging performance by the adherence of toners on the photoconductor to the charging roller: Especially, by the above-described exuding, the toners easily adhere onto the charging roller.
4. The component of the charging roller easily adheres onto the photo conductor.
5. The permanent deformation of the charging roller which is caused when stopping the photoconductor for a long period of time.
In order to solve the above problems, a close charging method, which brings a charging roller closer to a photoconductor, is proposed (refer to, for example, JP S63-149668A, JP H01-211779A, JP H01-267667A, JP H03-240076A, JP H04-358175A, and JP H05-107871A).
In the close charging method, the distance of closest approach (hereinafter, referred to as a space) between the charging roller and the photoconductor is set to 50 μm to 300 μm. If a voltage is applied to the charging roller in a state in which the charging roller faces the photoconductor, the photoconductor is charged. In this close charging method, since the charging unit does not have contact with the photoconductor, the above-described problems 4, 5 of the contact charging method are solved. Due to the above-described problem 3, the amount of toners which adhere onto the charging roller is reduced, so the close charging method is advantageous.
A property required for the charging roller for use in the close charging method is different from that for the charging roller for use in the contact charging method.
A general charging roller for use in the contact charging method has a structure in which a cored bar is covered with an elastic body such as a vulcanized rubber. In this contact charging method, it is required that the charging roller uniformly have contact with the photoconductor, in order to uniformly charge the photoconductor.
In the close charging method, when the charging roller formed by such an elastic body is used, the following problems are caused.
1. It is necessary to dispose space holding members such as spacers in both sides of the charging roller, respectively, in order to form a space between the photoconductor and the charging roller. However, since the charging roller is made of the elastic body, it is difficult to uniformly maintain the space because of the deformation of the elastic body. As a result, displacement in the charged potential and an uneven image resulting from the displacement are caused.
2. The vulcanized rubber material which forms the elastic body deteriorates with age and easily deforms. Accordingly, the size of the space changes over time.
In order to solve the above problems, it is considered to use a thermoplastic resin which is a non-elastic body. Thereby, the space between the photoconductor and the charging roller can be uniformly maintained.
It is known that the charging mechanism to the surface of the photoconductor by the charging roller is a discharge mechanism according to Paschen's Law by micro-discharge between the charging roller and the photoconductor. It is necessary to control the resistance value of the thermoplastic resin in a semi-conductive range (about 106 Ωcm-109 Ωcm), in order to maintain the photoconductor at a predetermined charged potential.
As a method of controlling this electric resistance value, a method of dispersing a conductive pigment such as a carbon black in a thermoplastic resin is known. However, if the thermoplastic resin (resistance adjusting layer) is set in a semi-conductive property range by using the conductive pigment, the variations in the resistance values are increased. As a result, a charging error is partially caused, which causes an image error.
On the other hand, as another method of controlling an electric resistance value, it is considered to use an ion-conductive material. Since the ion-conductive material disperses in a matrix resin on the molecular level, compared to the case when the conductive pigment is used, the variations in the resistance value are decreased. In this case, a partial charging error is not a problem relative to an image quality. However, a low-molecular-weight ion-conductive material such as an electrolyte salt has a property which easily bleeds out on the surface of the matrix resin. For this reason, the toners are firmly fixed onto the surface of the charging roller when bleeding out, resulting in an image error.
In order to avoid this bleeding out, it is considered to use a solid high-monocular form ion-conductive material such as a polyamide series elastomer or a polyolefin block polymer. In this case, the ion-conductive material disperses and fixes in the matrix resin, so that it hardly bleeds out on the surface. By only using the high-molecular form ion conductive material, the resistance-adjusting layer can not be controlled in the semi-conductive property range because the resistance value of the resistance-adjusting layer is high. For this reason, a method of applying a conductive property by adding an electrolyte salt is used. Such an electrolyte salt includes a perchlorate such as a sodium perchlorate or a lithium perchlorate, an organic phosphonium salt, or a fluorine-containing organic anion salt such as a trifluoromethanesulfonate lithium is used.
However, in the high-molecular form ion conductive material, since water from the air meditates in a conductive path, the water absorption property of the material itself is generally high, and the volume expansion degree (swelling property) by the water absorption is high. Accordingly, when the high-monocular form ion conductive material is used as the resistance-adjusting layer of the charging roller in the close charging method, the environmental variations of the space between the charging roller and the photoconductor are increased, and the charging performance is decreased, resulting in an image error. More particularly, since the charging roller expands in high-temperature and high-humidity environments, the size of the space between the charging roller and the photoconductor is decreased, and the charging roller may have contact with the photoconductor in an extreme case. In this case, since the discharge product on the photoconductor adheres onto the charging roller, the conductive property of that portion is lowered, resulting in an image error. On the other hand, since the size of the space is increased in low-temperature and low-humidity environment, the discharge from the charging roller to the photoconductor becomes uneven, resulting in an image error.
In order to reduce the swelling property of the charging roller, the blending quantity of the insulating thermoplastic resin is increased in the resistance-adjusting layer, or the functional group ratio, which contributes to the water absorption property in the high-molecular form ion-conductive material, is adjusted. Thereby, the swelling property can be reduced by the low water absorption of the material. In this case, the resistance is also increased, so that the conductive property required for the charging roller can not be obtained.